Crystalline silicon photovoltaic (PV) modules currently, as of 2012, account for approximately over 85% of the overall global PV anneal demand market and cumulative installed capacity. The manufacturing process for crystalline silicon PV is based on the use of crystalline silicon solar cells, starting with mono-crystalline or multi-crystalline silicon wafers. Non-crystalline-silicon-based thin film PV modules (e.g., such as CdTe, CIGS, and amorphous silicon PV modules) may offer the potential for low cost manufacturing process, but typically provide much lower conversion efficiencies (in the range of single digit up to about 14%) for commercial thin-film PV modules compared to the mainstream crystalline silicon PV modules (which provide efficiencies in the typical range of 14% to over 20% for commercial crystalline silicon modules), and an unproven long-term track record of field reliability compared to the well-established crystalline silicon solar PV modules. The leading-edge crystalline silicon PV modules offer the best overall energy conversion performance, long-term field reliability, non-toxicity, and life cycle sustainability among various PV technologies. Moreover, recent progress and advancements have already driven the overall manufacturing cost of crystalline silicon PV modules to below $0.80/Wp. Disruptive monocrystalline silicon technologies—such as high-efficiency thin monocrystalline silicon solar cells fabricated based on the use of reusable crystalline silicon templates, thin (e.g., ≤50 μm) epitaxial silicon, thin silicon support using backplane lamination, and porous silicon lift-off technology—offer the promise of high-efficiency (with solar cell and/or module efficiencies of at least 20%) and PV module manufacturing cost at well below $0.50/Wp at mass manufacturing scale.
FIG. 1A is a schematic showing the equivalent circuit of a typical solar cell, such as a crystalline silicon solar cell or a compound semiconductor such as a GaAs solar cell. A solar cell may be represented as a current source, producing the photo-generation current shown as IL or also known as short circuit current Isc (the current that flows when the solar cell terminals are shorted), in parallel with a diode, also in parallel with a shunt resistance, and in series with a series resistance. The current produced by the current source depends on the level of sunlight irradiation power intensity on the solar cell. Undesirable dark current ID flows in the opposite direction of IL and is produced by recombination losses in the solar cell. Voltage across the solar cell when its terminals are open and not connected to any load is known as Voc or open-circuit voltage. A realistic solar cell equivalent circuit also includes the finite series resistance Rs and the finite shunt resistance RSH, as shown in the circuit schematic of FIG. 1B. In an ideal solar cell, the series resistance RS is zero and the shunt resistance RSH is infinite. However, in actual realistic solar cells, the finite series resistance is due to the fact that a solar cell has parasitic series resistance components in its semiconductor and metallization (i.e., it is not a perfect conductor). Such parasitic resistance components, including semiconductor layer resistance and metallization resistance result in ohmic losses and power dissipation during the solar cell operation. The shunt resistance is caused by the undesirable leakage of current from one terminal to the other due to effects such as areal and edge shunting defects as well as other non-idealities in the solar cell. Again, an ideal solar cell would have a series resistance of zero and a shunt resistance of infinite resistance value.
FIG. 2A is again a schematic showing an equivalent circuit model of the solar cell, showing the current source, photo-generated current, and dark current (but not showing the parasitic series and shunt resistances), and FIG. 2B is a corresponding graph showing the typical current-voltage (IV) characteristics of a solar cell such as a crystalline silicon solar cell, with and without sunlight illumination on the cell. IL and ID are the desirable active photo-generated current and the undesirable dark current of the solar cell, respectively.
Solar cells used in PV modules are essentially photodiodes—they directly convert the sunlight arriving at their surface to electrical power through photo-generated charge carriers in the semiconductor absorber. In a module with a plurality of solar cells, any shaded cells cannot produce the same amount of electrical power as the non-shaded cells within the PV module. Since all the cells in a typical PV module are usually connected in series strings, differences in power also cause differences in photo-generated electrical currents through the cells (shaded vs. non-shaded cells). If one attempts to drive the higher current of the series-connected non-shaded cells through a shaded (or partially shaded) cell which is also connected in series with the non-shaded cells, the voltage of the shaded cell (or partially shaded cell) actually becomes negative (i.e., the shaded cell effectively becomes reverse biased). Under this reverse bias condition the shaded cell is consuming or dissipating significant power instead of producing power. The power consumed and dissipated by the shaded or partially shaded cell will cause the cell to heat up, creating a localized hot spot where the shaded cell is located, and eventually possibly causing cell and module failure, hence creating major reliability failure problems in the field.
A standard (i.e., typically a PV module comprising 60 solar cells) crystalline silicon PV module is typically wired into three 20-cell series-connected strings within the module, each protected by an external bypass diode (typically either a pn junction diode or a Schottky diode) placed in an external junction box which are electrically connected in series to each other to form the final PV module assembly electrical interconnections and the output electrical leads of the series-connected module. As long as the PV module receives relatively uniform solar irradiation on its surface, the cells within the module will produce nearly equal amounts of power (and electrical current), with a cell maximum-power voltage or Vmp on the order of approximately ˜0.5 V to 0.6 V for most crystalline silicon PV modules. Hence, the maximum-power voltage or Vmp across each strong of 20 cells connected in series will be approximately on the order of 10 to 12 V for a PV module using crystalline silicon cells. Under the uniform module illumination condition, each external bypass diode will have about −10 to −12 V reverse bias voltage across its terminals (while the module operates at its maximum-power point or MPP) and the bypass diode remains in the OFF state (hence, there no impact on the module power output by the reverse biased external bypass diodes in the junction box). In the case where a cell in a 20-cell string is partially or fully shaded, it produces less electrical power (and less current) than the non-shaded cells. Since the cells in the string are usually connected in series, the shaded solar cell becomes reverse biased and starts to dissipate electrical power, and therefore, would create localized hot spot at the location of the reverse-biased shaded cell, instead of producing power. Unless appropriate precautions are taken, the power dissipation and the resulting localized heating of the shaded cell may result in poor cell and module reliability due to various failure modes (such as failure of the reverse-biased shaded cell, failure of cell-to-cell interconnections, and/or failure of the module lamination materials such as the encapsulant and/or backsheet), as well as potential fire hazards in the installed PV systems.
Crystalline silicon modules often use external bypass diodes in order to eliminate the above-mentioned hot-spot effects caused by the partial or full shading of cells, and to prevent the resulting potential module reliability failures. Such hot-spot phenomena, which are caused by reverse biasing of the shaded cells, may permanently damage the affected PV cells and even cause fire hazards if the sunlight arriving at the surface of the PV cells in a PV module is not sufficiently uniform (for instance, due to full or even partial shading of one or more cells). Bypass diodes are usually placed on sub-strings of the PV module, typically one external bypass diode per sub-string of 20 solar cells in a standard 60-cell crystalline silicon solar module with three 20-cell sub-strings (this configuration may be one external bypass diode per sub-string of 24 solar cells in a 72-cell crystalline silicon solar module with three 24-cell sub-strings; many other configurations are possible for modules with any number of cells). This connection configuration with external bypass diodes across the series-connected cell strings prevents the reverse bias hot spots and enables the PV modules to operate with high reliability throughout their lifetime under various real life shading or partial shading and soling conditions. In the absence of cell shading, each cell in the string acts as a current source with relatively matched current values with the other cells in the strong, with the external bypass diode in the sub-string being reversed biased with the total voltage of the sub-string in the module (e.g., 20 cells in series create approximately about 10V to 12 volt reverse bias across the bypass diode in a crystalline silicon PV system). With shading of a cell in a strong, the shaded cell is reverse biased, turning on the bypass diode for the sub-string containing the shaded cell, thereby allowing the current from the good solar cells in the non-shaded sub-strings to flow in the external bypass circuit. While the external bypass diodes (typically three external bypass diodes included in the standard mainstream 60-cell crystalline silicon PV module junction box) protect the PV module and cells in case of shading of the cells, they can also actually result in significant loss of power harvesting and energy yield for the installed PV systems.
FIGS. 3A and 3B are diagrams of representative 60-cell crystalline silicon solar module with three 20-cell sub-strings 2 (with 20 cells in each sub-string connected in series) connected in series, and three external bypass diodes 4 to protect the cells during shading or excessive partial shading of any cells in the module (FIG. 3A shows single-cell shading, shaded cell 6, and FIG. 3B shows multi-cell partial shading conditions, partially shaded row 8). As an example, FIG. 3A shows a 60-cell module with 1 shaded cell in the bottom row (one 20-cell sub-string affected by shading) and FIG. 3B shows a 60-cell module with 6 partially shaded cells in the bottom row (three 20-cell sub-strings affected by shading). If one or more cells are shaded (or partially shaded to a significant degree of shading) in a sub-string (as shown in FIG. 3A), the bypass diode for the sub-string with the shaded cell(s) is activated and shunts the entire sub-string, thus both protecting the shaded cell(s) by preventing the hot spots and also reducing the effective module power output by about ⅓ (if only one sub-string out of three is affected by shading). If at least one cell per sub-string is shaded (as shown in FIG. 3B), all three bypass diodes are activated and shunt the entire module, thus preventing extraction of any power from the module when there is at least one shaded cell in each of the three 20-cell sub-strings.
As an example, a typical external PV module junction box may house three external bypass diodes in a 60-cell crystalline silicon solar module. The external junction box and related external bypass diodes contribute to a portion of the overall PV module Bill of Materials (BOM) cost and may contribute about 10% of the PV module BOM cost (i.e., as a percentage of the PV Module BOM cost excluding the cost of solar cells). Moreover, the external junction box may also be a source of field reliability failures and fire hazards in the installed PV systems. While most current crystalline silicon PV modules predominantly use external junction boxes with external bypass diodes placed in the junction box, there have been some examples of PV modules with front-contact cells placing and laminating the three bypass diodes directly within the PV module assembly, but separate from the front-contact solar cells, during the module lamination process (however, still using one bypass diode per 20-cell sub-string of front-contact cells). This example still has the limitations of external bypass diodes, i.e., even when a single cell is shaded the bypass diode shunts the entire substring of cells with the shaded cell within the sub-string thus reducing the power harvesting and energy yield capability of the installed PV system.
One known method to minimize the reliability failure effects of shading on a module in a series string of modules is to use bypass diodes across modules connected in series, the effect of which is shown in FIGS. 4A and 4B and an example circuit is depicted in FIG. 5. This is in effect the same as the modules with external bypass diodes within each module junction box. FIG. 4A shows a non-shaded current path for a solar cell module series and FIG. 4B shows the same solar cell module series with one module shaded and a bypass diode providing an alternative current path. And FIG. 5 is a schematic circuit model diagram of series-connected solar cells with an external bypass diode used in a module sub-string or string (each solar cell shown with its equivalent circuit diagram). If none of the cells are shaded, the bypass diode remains in the reverse bias state and the solar cell string operates normally, contributing fully to the solar module power generation. If any of the cells are partially or fully shaded, the shaded cell is reverse biased and the bypass diode is forward biased, hence, minimizing the possibility of a hot spot or damage to the shaded cell. In other words, when a module becomes shaded its bypass diode becomes forward biased and conducts current preventing performance degradation and reliability problems in the series string of modules. The bypass diode holds the voltage of the entire shaded module (or a sub-string with at least one shaded cell) to a small negative voltage (e.g., −0.5V to 0.7V) limiting overall power reduction in the module string array output.
FIG. 6 is a graph showing the current-voltage (I-V) characteristics of a crystalline solar cell with and without a bypass diode (example shown with a pn junction bypass diode). The bypass diode limits the maximum reverse bias voltage applied across a shaded solar cell to no more than the turn-on forward bias voltage of the bypass diode.
FIG. 7 is a diagram showing an example of a crystalline silicon PV module similar to that of FIGS. 4 and 5 with one shaded cell per 20-cell sub-string within a 60-cell module (such as shaded cell 10, three cells are shaded total) wherein the three shaded cells in the three 20-cell sub-strings result in the elimination of solar PV power provided by the module since all three 20-cell sub-strings are shunted by the bypass diodes to protect the shaded cells. Using an arrangement of one external bypass diode per 20-cell sub-string, the result of having three shaded cells in the three 20-cell sub-strings is that the power extracted from the PV module drops to zero even though only 3/60 of the module (or 3 out of 60 cells) is affected by shading. Again, this type of known PV module arrangement with external bypass diodes results in significant energy yield and power harvesting penalty for the installed PV systems in the field.
In crystalline silicon PV system installations with multiple module strings, the module shading effects and their detrimental impact on power harvesting and energy yield may be much larger than the examples shown above with a single series string of modules. In PV systems with multiple parallel strings of series connected module strings, the parallel strings must produce approximately the same voltage as one another (i.e., the voltages of parallel strings must be matched). As a result, the electrical constraint of having all module strings connected in parallel operating at approximately the same voltage does not allow a shaded string to activate its bypass diodes. Therefore, in many cases, shade on PV modules in one of the strings may actually reduce the power produced by the entire string. As a representative example, consider one non-shaded PV module string and one PV module string that is shaded as described in the previous example above. A Maximum-Power-Point-Tracking (MPPT) capability will enable the production of full power from the first PV module string and the production of 70% of full power from the second PV module string. In this way, both strings reach the same voltage (the currents from the parallel strings are additive at the same module string voltage for the parallel connected strings of series-connected modules). Therefore, in this example and using a centralized DC-to-AC inverter with centralized MPPT, the power produced by the PV module array would be 85% of the maximum possible power without any module shading.
FIGS. 8 and 9 are diagrams showing two examples of PV system installations. FIG. 8 shows example of a 3×6 array of PV modules (each with 50 W output) with bypass diodes connected to produce 600 V, 900 W PV output. FIG. 9 shows a series connection of 3 PV modules with bypass diodes and a blocking diode along with a charging battery. In conventional modules, module strings connected in series and in parallel may typically use bypass and blocking diodes. However, similar to previously described examples, these representative PV module installations suffer from the power harvesting limitation and reduced energy yield of the installed PV system due to the problems outlined earlier.
Another representative example of the monolithic integration of a bypass diode with a front-contact, compound semiconductor (III-V), multi junction solar cell for Concentrator PV (or CPV) applications. FIG. 10 is a diagram showing an example of monolithic integration of a bypass diode with a multi junction compound semiconductor CPV cell. This example shows a compound semiconductor Schottky diode used as monolithically integrated bypass diode on the same germanium (Ge) substrate as a compound semiconductor, multi junction solar cell for CPV applications. In this example, the Schottky bypass diode and the compound semiconductor, multi junction solar cell are both on the same side (top side) of the solar cell, and have different material layer stacks, thereby making the solar cell fabrication process much more complicated and costly (hence, such embodiment only demonstrated for the CPV application in which the CPV cells are quite expensive). As a result of monolithic integration of the Schottky bypass diode with the solar cell on the same expensive germanium substrate, the overall process complexity and cost are substantially and further increased while incurring an effective solar cell and solar panel efficiency penalty due to the integration of the Schottky bypass diode on the same side as the active sunnyside of the cell. This monolithic integration of the bypass Schottky diode on a front-contact compound semiconductor multi junction solar cell requires different stacks of material layers in the solar cell and in the bypass switch, hence, substantially complicating the overall monolithic solar cell processing, increasing the number of solar cell fabrication process steps, and raising its manufacturing cost. While such significant added processing complexity and cost increase for fabrication of the solar cell may be acceptable in a CPV solar cell, it cannot be economically viable in a non-very high concentration-CPV solar cell such as in crystalline silicon solar cells. FIG. 11 is a diagram showing an example of monolithic integration of a bypass diode with a multi junction compound semiconductor CPV cell. This example shows a pn junction diode used as monolithically integrated bypass diode on the same germanium (Ge) substrate as a compound semiconductor, multi junction solar cell. In this example, the pn junction bypass diode and the compound semiconductor, multi junction solar cell are both on the same side (top side) of the solar cell, and have different material stacks thereby making the solar cell fabrication process much more complicated and costly (hence, such embodiment only demonstrated for the CPV application in which the CPV cells are quite expensive). As a result of monolithic integration of the pn junction bypass diode with the solar cell on the same expensive germanium substrate, the overall process complexity and cost are and further increased while incurring an effective solar cell and solar panel efficiency penalty due to the integration of the bypass diode on the same side as the active sunnyside of the cell. Again, this monolithic integration of the bypass pn junction diode on a front-contact compound semiconductor multi junction solar cell requires different stacks of material layers in the solar cell and in the bypass switch, hence, substantially complicating the overall monolithic solar cell processing, increasing the number of solar cell fabrication process steps, and raising its manufacturing cost. While such significant added processing complexity and cost increase for fabrication of the solar cell may be acceptable in a CPV solar cell, it cannot be economically viable in a non-very high concentration-CPV solar cell such as in crystalline silicon solar cells.
In general, while the monolithic integration of the bypass diode (Schottky diode or pn junction diode) as shown on an expensive multi junction solar cell for very high concentration CPV applications may be acceptable for that particular application despite the extra cost and added manufacturing process complexity of the monolithic integration with the solar cell, the approaches demonstrated for the expensive compound semiconductor multi junction solar cells would be prohibitively too expensive and not acceptable for mainstream flat-panel (non-concentrating or low to medium concentration) solar PV cells and modules. Also, as noted previously, because the method of monolithic integration of the bypass diode consumes area otherwise used by the solar cell it reduces the effective sunlight absorption and hence the effective cell efficiency due to loss of sunlight absorption area.
Various solutions have been attempted to provide power harvesting and energy yield enhancement capability as compared to the more conventional capabilities of module-level DC-to AC micro-inverter power optimizer or module-level DC-to-DC converter power optimizer. One such technology utilizes programmable interconnects between the cells within the module in order to increase the energy yield of the cell-based PV module, for example Adaptive Solar Module (ASM) technology from Emphasis Energy. In some instances, this may enable a higher level of PV energy harvesting in the case of module shading compared to more traditional MPPT power optimizers. However, this technology utilizes a module level/external converter box (micro-inverter or DC-to-DC converter) and associated interconnects technology which may cost around $30 to over $100 per PV module. The module level converter box provides energy conversion from DC to DC or from DC to AC and may be built into the PV module assembly to provide reconfigurable or programmable cell interconnections within the module. However, the module level converter box is not and cannot be integrated with the individual cells, such as on cell backsides, and assembled with the individual cells.